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Crystal Structure of InTi0.75Fe0.25O3.375 and Phase Relations in the Pseudobinary System InFeO3-In2Ti2O7 at 1300 °C Yuichi Michiue,*,† Francisco Brown,‡ Noboru Kimizuka,‡ Mitsuko Onoda,† Masaki Nakamura,† Mamoru Watanabe,† Masahiro Orita,§ and Hiromichi Ohta§ National Institute for Research in Inorganic Materials, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan, Departamento de Investigaciones en Polimeros y Materiales, Universidad de Sonora, Rosales s/n, Hermosillo, Sonora, C.P. 83000, Mexico, R & D Center, Hoya Corporation, 3-3-1 Musashino, Akishima, Tokyo 196-8510, Japan Received March 3, 2000. Revised Manuscript Received May 4, 2000
An average structure of incommensurately modulated monoclinic InTi0.75Fe0.25O3.375 was determined using single-crystal X-ray diffraction. Unit cell parameters are a ) 5.9207(8), b ) 3.4249(5), c ) 6.3836(9), and β ) 107.96(1) with Z ) 2, and the space group is C2/m. The structure with pseudorhombohedral symmetry involves two metal positions: M1, practically occupied by In, and M2, by Ti and Fe. Two layers are alternately stacked along the c axis; one is an edge-shared M1O6 octahedral sheet and another consists of bipyramidal coordination at M2 sites. In the bipyramidal layer, a honeycomb lattice 68% occupied by oxygen ions surrounds the M2 site. These oxygen ions are subsequently displaced from their original positions to reduce mutual repulsion. This explains large displacement parameters U11 and U22 despite the normal U33 at the M2 site as well as at the oxygen site. Solid solution for the present phase is limited to InFeO3:In2Ti2O7 ) 2:3 at 1300 °C according to a phase relations study in the pseudobinary system InFeO3-In2Ti2O7. We propose a hypothetical compound with an unusual ABO3 structure based on a comparison of this compound and related structures.
Introduction Explorations of new complex metal oxides with simple molar ratios of constituents such as ABO3, ABO4, and AB2O4 are important. From the viewpoint of structural inorganic chemistry, ABO3 structures are classified into two groups.1 The first one contains A and B cations of suitable size for octahedral coordination and tends to adopt sesquioxide structures such as corundom or ilmenite. The second group has A cations comparable in size to oxygen ions and is made up of AO3 closestpacked layers and linked BO6 octahedra such as perovskites which represent an important commercial ceramics class. Giaquinta et al.2 have pointed out that InMnO3 consisting of alternating layers of InO6 octahedra and MnO5 trigonal bipyramids is unusual because it belongs to neither of the above structural families. According to their recent review of structural predictions,3 some other uncommon ABO3 structures, PbReO3,4 KSbO3,5,6 and LuMnO3,7 have been found so far. Con* To whom correspondence should be addressed. † National Institute for Research in Inorganic Materials. ‡ Universidad de Sonora. § Hoya Corporation. (1) Wells, A. F. Structural Inorganic Chemistry; Clarendon Press: Oxford, 1984. (2) Giaquinta, D. M.; zur Loye, H.-C. J. Am. Chem. Soc. 1992, 114, 10952. (3) Giaquinta, D. M.; zur Loye, H.-C. Chem. Mater. 1994, 6, 365. (4) Longo, J. M.; Raccah, R. M.; Goodenough, J. B. Mater. Res. Bull. 1969, 4, 191. (5) Hong, H. Y.; Kafalas, J. A.; Goodenough, J. B. J. Solid State Chem. 1974, 9, 345.
sidering the fact that InMnO3 is essentially isostructural to a high-pressure phase InGaO3,8 all the structure types were originally reported in the 1970s or earlier as well as well-known structures such as perovskite and corundum. Namely, no new ABO3 structure has been found in the past 20 years. Our synthetic study of ternary systems, In2O3-TiO2M2O3 (M ) Al, Ga, Cr, Fe) and In2O3-TiO2-MO (M ) Mg, Mn, Co, Ni, Cu, Zn) revealed two groups of orthorhombic (M ) Al, Ga, Fe, Cu) and monoclinic (M ) Al, Ga, Cr, Fe, Mg, Mn, Co, Ni, Cu, Zn) structures near the composition ratio In:(Ti + M) ) 1:1, called Unison-X1 phases.9,10 A single-crystal X-ray diffraction study of one of the orthorhombic forms, InTi0.67Fe0.33O3.33,11 showed that the structure has pseudohexagonal symmetry and is understood as a derivative of hexagonal InFeO312 isostructural to InMnO3. In this paper we report on monoclinic InTi0.75Fe0.25O3.375, which has a new struc(6) Goodenough, J. B.; Kafalas, J. A. J. Solid State Chem. 1973, 6, 493. (7) Geller, S.; Curlander, P. J.; Jefferies, J. B. Acta Crystallogr. 1975, B31, 2770. (8) Shannon, R. D.; Prewitt, C. T. J. Inorg. Nucl. Chem. 1968, 30, 1389. (9) Brown, F.; Flores, M. J. R.; Kimizuka, N.; Michiue, Y.; Onoda, M.; Mohri, T.; Nakamura, M.; Ishizawa, N. J. Solid State Chem. 1999, 144, 91. (10) Brown, F.; Kimizuka, N.; Michiue, Y.; Mohri, T.; Nakamura, M.; Orita, M.; Morita, K. J. Solid State Chem. 1999, 147, 438. (11) Michiue, Y.; Brown, F.; Kimizuka, N.; Watanabe, M.; Orita, M.; Ohta, H. Acta Crystallogr. 1999, C55, 1755. (12) Giaquinta, D. M.; Davis, W. M.; zur Loye, H.-C. Acta Crystallogr. 1994, C50, 5.
10.1021/cm000189d CCC: $19.00 © 2000 American Chemical Society Published on Web 07/11/2000
Structure of InTi0.75Fe0.25O3.375/Phases in InFeO3-In2Ti2O7
ture with pseudorhombohedral symmetry, and is not a distortion of the orthorhombic form. A hypothetical structure with a stoichiometric composition InFeO3, which may belong to unusual ABO3 structure types, is proposed, extrapolating from this nonstoichiometric phase. Interestingly, the proposed InFeO3 structure is described in the same manner as the hexagonal InFeO3 actually obtained: alternate stacking of two layers of InO6 octahedra and FeO5 trigonal bipyramids. Similarities and differences between the structures are discussed in detail. Although the structure should be incommensurately modulated judging from satellite spots, we dealt with the average structure in an analysis by main reflections only. Phase relations in the pseudobinary system InFeO3-In2Ti2O7 at 1300 °C were also studied and compared to those at 1100 °C.9 Experimental Section Phase Relations. Samples were prepared by a classical quenching technique. In2O3, TiO2, and Fe2O3 (99.9% or higher purity) heated at 850 °C for 1 day prior to reaction were mixed at a molar ratio of In2O3:TiO2:Fe2O3 ) 1:2x:1-x (1/3 e x e 6/7) using an agate mortar for about 30 min in ethanol. The mixture was pelletized (10 mm in diameter and 1.2 mm thick) and heated in an alumina crucible at 1300 °C for 2 days and then rapidly cooled to room temperature. After grinding, the sample was again pelletized and heated for 3 days. The weight loss of each sample was carefully checked after heating. Identification of the products and confirmation of an equilibrium state were done by powder X-ray diffraction measurements. Single-Crystal Analysis. A mixture of In2O3:TiO2:Fe2O3 ) 4:6:1 (in a mole ratio) was heated at 1350 °C for 2 days in air. Single crystals were grown by reheating the product at 1670 °C for 3 h and cooling to 1500 °C at 1 °C/min. Satellite reflections were observed at incommensurate positions analogous to polycrystalline samples prepared by solid-state reactions.9 Only main reflections were used for the analysis to determine the average structure. A brown prismatic crystal of 70 × 50 × 150 µm was mounted on a Rigaku AFC7R four-circle diffractometer with a graphite monochromator at 23 °C. Crystallographic data, experimental conditions, and parameters are listed in Table 1. Software package teXsan Version 1.9 (Molecular Structure Corporation, Rigaku Corporation. (1998)) was used for full matrix leastsquares refinement based on F2.
Results Phase Relations at 1300 °C. In phase relations in the pseudobinary system InFeO3-In2Ti2O7 at 1300 °C (Figure 1), Unison-X1 phases were obtained at InFeO3: In2Ti2O7 ) 2:3-2:1. Two structure types are known for Unison-X1, orthorhombic and monoclinic forms.9 The orthorhombic form has pseudohexagonal structure,11 which is regarded as a derivative of the hexagonal InFeO3 structure.12 The monoclinic structure is new, not a monoclinic distortion of the former as discussed in the following section. Hexagonal InFeO3 was unstable at 1300 °C as well as at 1100 °C.9 This compound was obtained by coprecipitation of In(OH)3 and Fe(OH)3 in a solution followed by heating at 700 °C in air.14,15 Therefore, three phases, In2O3, Fe2O3, and orthorhombic (13) Zachariasen, W. H. Acta Crsystallogr. 1967, 23, 558. (14) Nodari, I.; Alebouyeh, A.; Brice, J. F.; Gerardin, R.; Evrard, O. Mater. Res. Bull. 1988, 23, 1039. (15) Gerardin, R.; Aqachmar, E. H.; Alebouyeh, A.; Evrard, O. Mater. Res. Bull. 1989, 24, 1417.
Chem. Mater., Vol. 12, No. 8, 2000 2245 Table 1. Crystallographic Data and Conditions for Data Collection and Refinement for InTi0.75Fe0.25O3.375 formula weight crystal system space group a (Å) b c β (°) V (Å3) Z Dx (g/cm3) µ (Mo KR) (mm-1) radiation refinement of cell parameters scan mode 2θ max.
218.70 monoclinic C2/m 5.9207(8) 3.4249(5) 6.3836(9) 107.96(1) 123.14(3) 2 5.90 13.00 Mo KR (0.71069 Å) 25 reflections (40.6° e θ e 42.3°) ω-2θ 100° (0e he 12, 0 e k e 7, -13 e l e 13) standard reflections 3 every 200 (variation within 2.5%) reflections measured 1380 reflections used for calculation 722 absorption correction experimental (ψ-scan) transmission factor 0.996-0.575 a extinction coefficient 6.5(3) × 10-6 final R1, wR2b 0.0288, 0.0538 (weight factor: w ) 1/σ(Fo)2) ∆Fmin, ∆Fmax -1.58, 1.47 (e/Å3) a Type 2 Gaussian isotropic (ref 13). b R ) Σ||F | - |F ||/Σ|F | 1 o c o for I > 2σ(I) and wR2 ) (Σw(Fo2 - Fc2)2/ΣwFo4)1/2.
Figure 1. Phase relations at 1300 °C. Filled circles, one phase exists; open circles, two phases coexist; triangles, three phases coexist. Unison-X1 phases are on the thick line in the pseudobinary system InFeO3-In2Ti2O7. The structure of the monoclinic Unison-X1 is new, not a distortion of the orthorhombic form.
Unison-X1, coexisted in the InFeO3-rich side out of the Unison-X1 range in Figure 1. No compound with the composition In2Ti2O7 was obtained at 1100,9 1300, or 1350 °C,16 even though the ionic radius of In3+ in eight coordination (0.923 Å) is located between those of Sc3+ (0.87 Å) and Lu3+ (0.97 Å),17 constituents of pyrochlore compounds Sc2Ti2O7 and Lu2Ti2O7.16 In2TiO5,18 TiO2, and monoclinic Unison-X1 therefore coexisted in the In2Ti2O7-rich side. Phase relations at 1300 °C (Figure 1) resemble those at 1100 °C. Note, however, that the monoclinic form is limited to InFeO3:In2Ti2O7 ) 2:3 at 1300 °C, while a wide range of InFeO3:In2Ti2O7 ) 2:3(16) Brixner, L. H. Inorg. Chem. 1964, 3, 1065. (17) Shannon, R. D. Acta Crystallogr. 1976, A32, 751. (18) Senegas, J.; Manaud, J.-P.; Galy, J. Acta Crystallogr. 1975, B31, 1614.
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Table 2. Fractional Coordinates, Occupation Ratios, and Displacement Parameters (Å2) for InTi0.75Fe0.25O3.375a position
occupancy
x
y
z
Beq
M1 (In/Ti) M2 (Ti/Fe/In) O1 O2
0.93(2)/0.07 0.690(8)/0.25/0.060 1 0.68(1)
0 0.5 0.6103(4) 0.663(1)
0 0 0 0.5
0 0.5 0.8332(4) 0.493(1)
0.623(5) 2.46(2) 1.07(4) 5.9(2)
position
U11
U22
U33
U13
M1 M2 O1 O2
0.0066(1) 0.0325(6) 0.014(1) 0.106(7)
0.0070(1) 0.0512(8) 0.017(1) 0.092(7)
0.0103(1) 0.0057(4) 0.009(1) 0.020(4)
0.00294(9) -0.0002(3) 0.0025(8) 0.010(4)
a Temperature factors take the form exp{-(h2β 2 2 4 2 11 + k β22 + l β33 + 2hkβ12 + 2hlβ13 + 2klβ23)}. Beq ) ( /3)ΣiΣj βijaiaj. Uij ) βij/2π ai*aj*. U12 and U23 are 0 for all atoms.
Table 3. Selected Data and Parameters Based on Structure Models model M1 site: In Ti Fe Beq (Å2) M2 site: In Ti Fe Beq (Å2) O2 site: O Beq (Å2) R1 (%) wR (%) chemical formula total charge
1
2
3
4
5
1
1
0.903(4)
0.91(2)
0.595(6)
0.620(5)
0.625(5)
0.09 0.615(4)
0.623(5)
0.75 0.25 2.00(3)
0.27(2) 0.73 2.38(2)
0.75 0.25 2.37(2)
0.065 0.75 0.185(9) 2.45(2)
0.060 0.690(8) 0.25 2.46(2)
0.6875 6.2(3) 3.81 7.33 InTi0.75Fe0.25O3.375
0.70(1) 5.9(2) 2.88 5.43 InTi0.27Fe0.73O3.40
0.67(1) 6.1(2) 3.05 5.44 In0.903Ti0.75Fe0.25O3.34
0.68(1) 5.8(2) 2.95 5.28 In0.975Ti0.75Fe0.275O3.36
0.68(1) 5.9(2) 2.88 5.38 In0.99Ti0.76Fe0.25O3.36
0
-0.53
-0.22
+0.03
+0.04
1:1 was seen for the monoclinic form at 1100 °C.9 In other words, the orthorhombic form expanded and the monoclinic form contracted at 1300 °C compared to 1100 °C, implying that the orthorhombic/monoclinic boundary shifts toward In2Ti2O7 with increasing temperature. Structure Determination of InTi0.75Fe0.25O3.375. Fractional coordinates, occupancies, and displacement parameters of InTi0.75Fe0.25O3.375 (monoclinic Unison-X1) are listed in Table 2. Great care was paid in determining occupation ratios at the metal (M1 and M2) and O2 sites in this nonstoichiometric compound. At initial stages of the refinement, the M1 site was allotted for In and the M2 site was for Ti and Fe, analogous to InTi0.67Fe0.33O3.33 (orthorhombic Unison-X1).11 An occupation factor at the O2 site was fixed to keep charge neutrality of the whole crystal. The results are shown as model 1 in Table 3, which was taken as a basic model and modified as follows. Refinement of the Ti/Fe ratio at the M2 site and the occupation factor at the O2 site resulted in considerable improvement of reliability factors (model 2). The obtained Ti/Fe ratio appears, however, unreasonable for the monoclinic phase, judging from phase relation studies. The charge discrepancy between cations (metal ions) and anions (oxygen ions) was significant because the occupation factor at the O2 increased little despite the considerable decrease in positive charges due to the decrease in the Ti4+/Fe3+ ratio at the M2 site. We tried another model (model 3) by introducing vacancies at the M1 site which gave similar R values to those in model 2. Charge neutrality was poor, however, because the decrease in the occupation factor at the O2 site was too small to compensate for the decrease in positive charge due to the introduction of
0.93(2) 0.07
In vacancies. These results imply, first, electrons in model 1 should decrease at the M1 site and/or increase at the M2 site, and second, total cation charge (i.e., metal ions) should not differ greatly from those in model 1 because anions (i.e., oxygen ions) seem rather independent of occupation ratios at metal sites, as is shown by models 2 and 3, giving roughly similar occupation ratios at the O2 site. These requirements are satisfied by (i) combining Fe substitution for part of In at the M1 site and In substitution for part of Fe at the M2 site (model 4), or (ii) combining Ti substitution for part of In at the M1 site and In substitution for part of Ti at the M2 site (model 5). Fitting was equally good for both models (Table 3). Charge neutrality was also fairly good. Although the two models are equally acceptable at this stage, model 5 was taken as the final model (Table 2) because Ti substitution is commonly applicable to other isostructural compounds, InTixM1-xO3+x/2 (M ) Al, Ga, Cr, etc.) actually synthesized.10 The chemical composition of the crystal, which may deviate from that of the starting mixture due to the evaporation of certain species in crystal growth, could not be determined precisely because of the inhomogeneity of the crystal growth product and too much fluctuation in electron probe microanalysis. We used the chemical composition of the starting mixture, InTi0.75Fe0.25O3.375, as a nominal composition because few deviations from this were seen in models 4 and 5. Note that occupation factors at the M2 and O2 sites may have been overestimated, leading to large displacement parameters at both sites, but this was ruled out because in model 2 the Ti/Fe ratio refinement at the M2 site increased electrons at this site (i.e., decreased the Ti/Fe ratio) with little change
Structure of InTi0.75Fe0.25O3.375/Phases in InFeO3-In2Ti2O7
Chem. Mater., Vol. 12, No. 8, 2000 2247
Figure 2. (a) Structure of InTi0.75Fe0.25O3.375 projected along a direction slightly tilted from the b axis. The M1 site is at about the center of the M1O6 octahedron. (b) Atomic arrangement in the slab at z ≈ 1/2. The occupation ratio at the O2 site is 0.68. (c) Example of probable local arrangements in the slab at z ≈ 1/2. Small filled circles are metal ions, large shaded circles oxygen ions, and dotted circles vacancies. Arrows indicate probable positional displacement of oxygen ions from their original positions.
Figure 3. Idealized rhombohedral structure of InTi0.75Fe0.25O3.375 (monoclinic Unison-X1). (a) Projection along [1 1 0] (in hexagonal axes). am and cm are axes of the monoclinic cell. (b) Closest packing sites, A, B, and C, projected along [0 0 1]. am and bm are axes of the monoclinic cell. (c) 12 layers projected along [0 0 1]. The occupation ratio is 0.68 at the O2 site.
in the equivalent displacement parameter Beq. Occupation factors and displacement parameters at the O2 site were roughly similar among the five models. Explanations for unusually large displacement parameters at these sites will be given in the following paragraph. The structure with pseudorhombohedral symmetry is formed by alternate stacking of two layers: one is made up of edge-shared M1O6 octahedra and another consists of bipyramidal coordination around the M2 site (Figure 2a). The octahedral layer is common, while some peculiarity is seen in the bipyramidal, which contains axial M2-O1 bonds and in-plane M2-O2 bonds. The
O2 site in the slab at z ≈ 1/2 forms a honeycomb lattice surrounding the M2 (Figure 2b). The occupation ratio at the O2 site is 0.68, meaning that 4.1 ()0.68 × 6) positions among 6 around one metal ion are actually occupied on the average by oxygen ions. Primary images of local structures and coordination features in this slab are given from present data, although a complete structure analysis including satellite reflections is necessary for accurate discussion. In an example of probable local arrangements in the slab (Figure 2c), 44 of 64 sites were occupied by oxygen atoms and the rest vacant. Oxygen ions may subsequently be displaced
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Figure 4. (a) Idealized hexagonal structure of InTi0.67Fe0.33O3.33 (orthorhombic Unison-X1) projected along [1 1 0]. (b) 8 layers projected along [0 0 1]. The occupation ratio is 0.74 at the O2 site and 0.59 at the O3. Table 4. Interatomic Distances for InTi0.75Fe0.25O3.375 atom M1 average M2
atom
multiplicity
distance (Å)
O1 O1
×4 ×2
O1 O2 O2
×2 ×4 ×2
2.218(2) 2.223(2) 2.220 2.023(3) 1.974(4) 1.980(8) 1.988
average
from their original positions to reduce mutual repulsion (arrows, Figure 2c). This explains large displacement parameters U11 and U22 despite normal U33 at the M2 site as well as at the O2 site (Table 2), similar to metal (Ti/Fe) and oxygen ions in orthorhombic Unison-X1, InTi0.67Fe0.33O3.33.11 Selected interatomic distances are listed in Table 4. In-plane M2-O2 bonds are longer than those of related structures, InTi0.67Fe0.33O3.33 (1.94 Å average11) and InFeO3 (1.9208(1) Å).12 This is concerned with the expansion of the rectangle defined by unit cell dimensions, a × b: 20.278 Å2 for InTi0.75Fe0.25O3.375, 19.547 Å2 for InTi0.67Fe0.33O3.33, and 19.172 Å2 for InFeO3 (in the orthohexagonal cell). The variation in ab area coincides with the fact that oxygen ions on the M2-O2 plane are most condensed in InTi0.75Fe0.25O3.375 and the least in InFeO3. The more oxygen ions in the M2-O2 plane, the larger the ab area for reducing electrostatic repulsion between oxygen ions, elongating in-plane M2-O2 distances. Discussion The structure of InTi0.75Fe0.25O3.375 (monoclinic UnisonX1) is closely related to that of InTi0.67Fe0.33O3.33 (orthorhombic Unison-X1) because the orthorhombic form is also constructed by alternate stacking of InO6 octahedral and (Ti/Fe)O5+R bipyramidal layers.11 The two structures differ essentially in filling by metal and oxygen ions compared to Figures 3 and 4, where structural distortion is ignored and ideal symmetry is used for simplicity: a rhombohedral lattice for the monoclinic and a hexagonal lattice for the orthorhombic. Minor components at metal sites, Ti at the M1 site and In at the M2 site, are ignored in Figure 3. All metal and oxygen ions are positioned at the three sites, A, B, and C, defined by closest packing (Figure 3b). The rhombohedral form consists of 12 layers, R1, R2, ..., R12,
Figure 5. Relationships between layers forming structures.
stacked along the c axis (in a hexagonal setting, Figure 3c), while the hexagonal form consists of 8 layers, H1, H2, ..., H8 (Figure 4b). The first four layers R1-R4 in the rhombohedral form are identical to those in the hexagonal, H1-H4 (Figure 4b), except that occupation ratios at oxygen sites differ slightly between R3 and H3. Other layers are made from these four layers by simple rules differing between the two structures (Figure 5). In rhombohedral stacking, ions at A sites in the nth layer go to B sites in the (4+n)th layer, and those at B sites go to C sites in the latter, while those at C sites in the former go to A sites in the latter. Indium ions occupy A sites in layer R1, B sites in R5, and C sites in R9. Indium positions are described by a repetition of |ABC|ABC|... along the c axis. Oxygen ions at C sites in
Structure of InTi0.75Fe0.25O3.375/Phases in InFeO3-In2Ti2O7
layer R2 occupy A sites in R6 and then B sites in R10. Ti and Fe ions are at C sites in layer R3, A sites in R7, and B sites in R11. The unit of repetition for Ti and Fe is |CAB|. Another rule is applied for the hexagonal form. Ions at A sites in the nth layer remain at A sites in the (4+n)th layer, but those at B sites in the former go to C sites in the latter, and those at C sites in the former go to B sites in the latter. Indium ions on layers H1 and H5 are always at A sites. Oxygen ions at C sites in layer H2 occupy B sites in H6. Ti and Fe ions are at C sites on layer H3 and B sites on H7, but never occupy A sites. Metal positions (i.e., In, Ti, and Fe) are described by a repetition of |ACBACB |ACBACB|... along the c axis in rhombohedral stacking, while |ACAB|ACAB|... in the hexagonal. This difference leads to rather different diffraction patterns for powder samples of the two phases. At first sight, diffraction patterns of monoclinic Unison-X1 resemble those of fluorite-related structures such as pyrochlore because the fluorite structure has basically the same packing mode of metal ions to that of the rhombohedral form (Figure 3). As pointed out elsewhere,11 orthorhombic Unison-X1 can be regarded as a nonstoichiometric form of hexagonal InFeO3. This stoichiometric structure is isostructural to InMnO32 and consists of two layers of InO6 octahedra and FeO5 trigonal bipyramids. Layers H3 and H7 in the nonstoichiometric form (Figure 4b) are replaced by layers H3′ and H7′ (Figure 6a) in the stoichiometric form. Fe3+ ions are substituted for all Ti4+ ions in the nonstoichiometric form to give a stoichiometric compound. Excess oxygen ions were removed from H3′ and H7′ layers to keep the charge neutrality in the whole crystal. The same modification applied to monoclinic Unison-X1 gives a hypothetical compound with stoichiometric composition InFeO3 (rhombohedral InFeO3); layers R3, R7, and R11 in Figure 3c are replaced by layers R3′, R7′, and R11′ in Figure 6b. All Ti4+ ions in the nonstoichiometric form are replaced by Fe3+ ions, removing additive oxygen ions. Thus, two layers of InO6 octahedra and FeO5 trigonal bipyramids are seen and oxygen ions in Fe-O2 planes form a
Chem. Mater., Vol. 12, No. 8, 2000 2249
Figure 6. (a) Layers in the hexagonal InFeO3 structure. H3′ and H7′ are substituted for H3 and H7 in Figure 5. (b) Layers in the hypothetical InFeO3 structure with rhombohedral symmetry. R3′, R7′, and R11′ are substituted for R3, R7, and R11 in Figure 4.
triangle lattice in rhombohedral InFeO3. The stoichiometric structures, hexagonal and rhombohedral InFeO3, belong to unusual ABO3 types from the viewpoint of structural inorganic chemistry. InMnO32 and InGaO3 (under high pressure8) isostructural to hexagonal InFeO3 and similar structures (YAlO319 and RMnO3, Rd Lu, Yb, Er, etc.20,21) were synthesized, but not rhombohedral InFeO3 or isostructural compounds, so far. This suggests that the rhombohedral structure is more unstable than the hexagonal form and additive oxygen ions may have to be introduced into Fe-O2 planes to stabilize the rhombohedral structure. Attempts to synthesize rhombohedral InFeO3, a new ABO3 structure, are worth trying because all ABO3 structure types were originally reported in the 1970s or earlier. Supporting Information Available: Bond angles and tables of observed and calculated structure factors for monoclinic InTi0.75Fe0.25O3.375. This material is available free of charge via the Internet at http://pubs.acs.org. CM000189D
(19) Bertaut, F.; Mareschal, J. C. R. Acad. Sci. Paris 1963, 257, 867. (20) Yakel, H. L.; Koehler, W. C.; Bertaut, E. F.; Forrat, E. F. Acta Crystallogr. 1963, 16, 957.
(21) Isobe, M.; Kimizuka, M.; Nakamura, M.; Mohri, T. Acta Crystallogr. 1991, C47, 423.
